1. Field of the Invention
The present invention relates to the field of frequency converted solid state laser, and in particular to intracavity frequency conversion.
2. Description of the Related Art
Laser radiation at visible and UV wavelengths with high average power and repetition rates is useful for numerous industrial applications ranging from via hole drilling, laser texturing, micromachining, stereophotolithography, memory repair and direct writing. Medical applications including surgical and other therapeutic procedures can also benefit from availability of high power at shorter wavelengths, especially from compact and reliable devices.
In conventional solid state laser systems based on second, third or fourth harmonic generation, the output is typically produced in an extracavity module, using one or more nonlinear crystals. Generally, the art recognizes that the efficiency of the external harmonic conversion is limited by the available peak power of the fundamental laser and damage to the coatings of the nonlinear crystal. Most diode pumped laser systems focus the fundamental beam into the crystal to generate the peak intensities needed for high conversion. On the other hand, coatings applied to the crystal as well as to other optical elements in the harmonics path are known to be increasingly susceptible to damage as the power density or the fluence increases, hence the efficiency of the conversion process is limited by lifetime considerations. These limitations tend to become progressively more severe at higher harmonics because of the propensity for damage to coatings and even bulk materials to occur more readily at shorter wavelengths especially as the higher harmonics get deeper into the UV.
One approach to increasing powers available at harmonic wavelengths is to increase the peak power of the fundamental laser, thereby achieving the same conversion at lower pulse energies and relaxing focusing requirements of the beam incident on the crystal. This can be achieved by increasing pulse energies or by going to shorter pulses. In diode pumped systems, both of those options are subject to inherent limitations, especially for higher repetition rates. For many applications, repetition frequencies of interest are generally in excess of 1 kHz, and in some cases over 100 kHz. For such systems, requiring also higher energy outputs lead to average powers that are not consistent with stable TEM00 operation of the cavity due to thermal lens and stress aberration considerations. It is also known that for diode end-pumped lasers, the pulse-to-pulse stability is reduced and the pulse duration increases as the repetition rate is increased. Thus, both average and peak power scalability in diode end-pumped systems are limited, even as more diode pump power becomes available. For example, the maximum achievable single transverse mode power per rod is generally limited to about 30 W per rod from standard commercial diode end-pumped laser systems based on Nd:YVO4 or Nd:YAG lasers. Using two rods and additional pump ports has demonstrated pulsed power scaling of up to 50 W (see Hodgson et al, CLEO 2001 Proceedings, Paper CThC4) but this comes generally at the expense of longer, more complex cavities and longer pulse durations, as well as decreased output power stability at higher repetition rates. Correspondingly, the state-of-the-art for external frequency conversion of diode end-pumped lasers from practical TEM00 mode lasers has so far been limited to about 20–25 W for the green second harmonic and about 10 W for the UV third harmonic (see Hodgson et al ref. Above). Thus, even assuming optimized end-pumped configuration with up to 45–50% diode-to-TEM00 fundamental output, the 355 nm UV radiation is produced with generally less than 15% diode-to-UV optical efficiency.
It has been recognized by the known art that certain advantages and improvements to the harmonic conversion process could be obtained with an intra-cavity conversion architecture. Intracavity frequency doubling has been successfully implemented for CW and quasi-CW diode end-pumped solid state lasers. For example, the Millenia commercial system available from Spectra-Physics can produce output powers in excess of 6 W in the green using a fiber-coupled diode end-pumped Nd:YVO4 gain material and an intra-cavity doubling scheme, with a non-critically phase-matched (NCPM) LBO crystal. Techniques and structures for intra-cavity tripling into the UV for CW lasers have also been disclosed. See for example U.S. Pat. No. 6,241,720 to Nighan et al and U.S. Pat. No. 6,389,043 Nelte and Hargis, among others. Among high power diode pumped pulsed Intracavity doubled lasers, there are a number of commercial products available, for example a 50 W system from Lee lasers. However, most of the intracavity converted pulsed lasers available to date, have multi-mode beam quality, which provides a poor match to many industrial applications of interest.
The known art recognizes that the primary advantage of intra-cavity frequency conversion is the ability to rely on the high power circulating inside the laser resonator to provide harmonic conversion with higher overall efficiency than is possible in an extra-cavity configuration. One key benefit of cycling the power through the crystal is being able to achieve the desired conversion while limiting the power densities incident on the crystal. As a consequence, the crystals may exhibit generally longer lifetimes. As a second benefit, the power cycling through the crystal allows for improved pulse-to-pulse stability. The possibility that some of these advantages may be attendant to pulsed cavities including a Q-Switch were recognized nearly a decade ago, for example, by Dacquay in U.S. Pat. No. 5,191,588 and Wu in U.S. Pat. No. 5,278,852. Both of these early patents failed, however, to appreciate the difficulties inherent to using dichroically coated elements to extract and/or isolate the higher harmonics when the infrared fundamental laser beam is optically coupled and collinear with the generated harmonic radiation. Attempts to remedy these deficiencies were presented by Yin in U.S. Pat. No. 5,898,717 and by Alfrey in U.S. Pat. No. 6,002,695 which describe embodiments for UV extraction based on one or more Brewster cut prisms as beam isolation or output elements included in resonant cavities containing a gain medium and two or more nonlinear crystals used for the harmonic conversion process. Combinations of dichroic or trichroically coated elements cut near the Brewster surface were also shown in Yin's U.S. Pat. No. 6,061,370 directed to a fourth harmonic laser and U.S. Pat. No. 6,366,596 to Yin et al, which disclosed a diode-pumped laser with intracavity harmonic as well as parametric frequency conversion.
Alternative variations of intracavity conversion configurations included use of sub-resonators. For example, Zhou et al in U.S. Pat. No. 5,943,351 teaches use of sub-resonators for circulating the second harmonic and a variety of multi-coated optics for extracting a desired UV beam. As shown in this patent, the fundamental resonator and the harmonic sub-resonators are generally constructed linearly, so as to provide multiple passes through successively higher harmonic crystals. Also disclosed in this patent are intracavity UV reflecting mirrors which are useful in blocking the UV radiation from reflecting back towards the main cavity and other crystals b, However, Zhou et al failed to note that such constructions of harmonic sub-cavities require dichroic and trichroically coatings, which are known to be difficult to manufacture in practice and are generally more susceptible to damage than standard coatings, especially when subjected to high intensity UV beams. Improvements suggested by Yin in U.S. Pat. No. 6,327,281 provided for a sub-resonator only for the second harmonic and further including angled optics and dispersive surfaces to separate the different wavelengths, thereby obviating the need to place a highly reflective 355 nm mirror within the cavity. Unlike spectral separation which tends to be imperfect due to leakage of undesirable frequencies, the spatial separation means shown by Yin generally provide for nearly pure spectral content, as long as the cavity design allows for sufficient angular separation. In practice, the use of dispersive surfaces such as intra-cavity prisms or Brewster plates tend to lead to long asymmetric resonators with each additional such element adding to the design complexity. In high power laser operation, this may adversely affect the stability conditions for TEM00 operation and complicates the laser alignment. Longer resonators may also result in longer pulse durations than is desired for the applications contemplated.
An elegant approach to providing spatial beam separation with the fewest number of added optical elements was described by Grossman et al in U.S. Pat. No. 5,850,407 wherein a Brewster-cut tripler crystal was described which was uncoated on the sensitive exit side. This allows for sufficient spatial separation of the fundamental, green and UV beams without adding additional dispersive surfaces while reducing the number of required coatings especially on the sensitive exit face of the intracavity tripler crystal. Advantageously, this technique provides for a more compact resonator as compared to designs including one or more intracavity prisms, at the same time avoiding the need for lossy and damage susceptible dichroic and anti-reflective (AR) coatings. Including a Brewster cut crystal in the cavity results, however, in an elliptically-shaped cavity mode. Although such an elliptical beam can be made more circular through use of tilted, curved reflectors, cylindrical optics or a fused silica Littrow prism, including such optics in the cavity will complicate the overall resonator design, potentially negating the purpose of the original compact, readily aligned construction. The system with a Brewster cut tripler also does not readily extend to higher harmonics, being suited only to the specific harmonic for which it is designed.
Although recognizing the importance of stable TEM00 operation in intracavity converted lasers, the known art has also failed to address the ramifications of requiring operation in the stable regime in the presence of intracavity elements across a range of output parameters. In particular, analysis and discussions of the known art generally concentrated on various harmonic extraction schemes while neglecting due consideration of issues affecting pulsed lasers with power scaling capability in a practical setting.
Generally, it is well known in the art of solid state lasers that, as the pump power incident on a laser crystal is increased, thermal lensing becomes a limiting factor for diffraction limited operation. It is known for example that for gain media such as Nd:YVO4 and Nd:YAG the thermal lens becomes very strong as the power density is increased with focal lengths becoming as short as 10 cm at elevated power levels. Although such a strong lens can be compensated by clever cavity design, the aberrations in the lens eventually degrade the single mode performance of the laser.
Additionally, there are trade-offs between the pump spot size and laser beam mode size in optimizing a design for TEM00 operation. This in turn sets limits on the spot sizes that may be utilized in the nonlinear crystals, thus affecting the overall harmonic conversion efficiency. For example, the known art as exemplified in U.S. Pat. No. 6,366,596 to Yin et al teaches an intracavity tripled laser where the fundamental laser mode size is between 0.8 and 2.0 mm and the laser medium has a diameter of about 1.6 to 4 times the fundamental beam diameters. Setting the laser parameters in this manner is, however, neither necessary nor sufficient in terms of providing stable TEM00 operation over a desirable range of repetition rates at either low or high power.
One area of concern to intracavity converted lasers involves methods for extraction of the harmonic beam. In particular, it has already been recognized that beam separation is a particularly challenging aspect of any intracavity harmonic conversion process, especially for high power systems. This is a direct result of the fact that the intracavity conversion process generally involves collinearly coupled fundamental and harmonics co-propagating in at least a portion of the same cavity. Therefore, extracting one particular desirable wavelength while suppressing others will, as a rule, require more complex separation schemes as compared with those commonly utilized in externally converting systems. The difficulties are compounded at the high peak powers characteristic of pulsed resonators, and an intracavity pulsed system is known to be very demanding of the resonator optics, especially if any light in the UV portion of the spectrum is produced, as optics are generally become less damage resistant as the wavelength becomes shorter. Thus, circulating UV light, even in relatively small amounts can damage AR coatings including those protecting sensitive laser rod and nonlinear crystals.
Typically, optics used in known systems for beam separation involved dichroic and trichroic coatings or dispersive surfaces, each with its attendant disadvantages and challenges. Thus, dichroic or trichroic coatings with the requisite reflection and transmission properties can be difficult to design and they remain generally susceptible to damage. For example, coating a standard flat optic for high transmission in the green and UV and high reflection in the infrared is known to be a difficult problem. With all three wavelengths incident on the same spot, an imperfect coating can undesirably absorb some of the UV light, becoming “thermalized”. This can result not only in unpredictable power output drops but in general instability of the laser.
Accordingly, the known art, taken as a whole, has failed to consider aspects of intracavity conversion that are important to practical and readily manufacturable laser systems with output in one or more harmonics of a fundamental. Many issues associated with achieving these desirable characteristics concerned with intracavity conversion have been overlooked.